This new application addresses an important question in vascular biology which concerns how newly formed endothelial cell (EC)-lined tubes are stabilized. We have developed excellent models of this process in vitro and have identified key mechanisms and molecules, which are required for these events. EC-derived proteinases such as MMP-1 are capable of degrading 3D collagen matrices during tubular morphogenesis, which destabilizes tubes and induces regression. We hypothesize that EC supporting cells such as pericytes present proteinase inhibitors such as TIMP-3 and RECK which blocks the natural tendency of ECs during angiogenesis to degrade the extracellular matrix environment in which they are suspended. These pericyte-derived inhibitors likely represent new molecules that are critical to the stabilization of newly formed tubes. Preliminary data shows that TIMP-3 is unique in that it can completely inhibit EC tubular morphogenesis as well as stabilize EC-lined tubes. Furthermore, TIMP-3 is heavily expressed by primary cultures of pericytes and is not expressed by ECs. Inclusion of vascular smooth muscle cells (also express TIMP-3) with ECs in 3D models of MMP-1-dependent tube regression completely blocks the process by interfering with MMP-1 proenzyme activation. We will utilize a balanced experimental approach to determine the role of pericyte-derived proteinase inhibitors in EC tube stabilization in vitro and in vivo (using recombinant adenoviral gene delivery and TIMP-3 knockout mice). The mechanisms involved in how pericytes stabilize EC tubes during angiogenesis and vascular development remain unknown and this proposal will directly test the novel hypothesis that proteinase inhibitors are key molecules regulating these events. The specific aims of this application are; Aim #1. To investigate the primary mechanisms and molecules (i.e. proteinase inhibitors and growth factors) which regulate the ability of pericytes/vascular smooth muscle cells to induce capillary tube stabilization in vitro and in vivo. Aim #2. To investigate the molecular mechanism by which TIMP-3 regulates capillary tube stabilization in vitro and in vivo through complete inhibition of EC invasion and lumen development during tubular morphogenesis in three-dimensional matrices. Aim #3. To investigate the ability of human matrix metalloproteinase-1 (MMP-1) as well as mouse interstitial collagenases (e.g. MMP-13) to directly control capillary tube regression events in vitro and in vivo.